Measuring or monitoring the concentration of oxygen is important in areas in which the concentration may fall below safe levels. For example, the concentration of oxygen should be measured and monitored in an industrial plant in which workers are handling asphyxiants such as nitrogen or carbon dioxide, to ensure that the leakage of the asphyxiants into the work area does not cause the oxygen concentration to fall below a safe level. The safe level is typically between the atmospheric concentration of oxygen, which is 21% and a minimum concentration of 19.5%. Otherwise, the workers may be harmed, sometimes fatally, as a result of a lack of oxygen. Similarly, when maintenance workers are entering an enclosed or unventilated space such as a storage tank, sewer, or utility tunnel, it is necessary to measure the concentration of oxygen before entry. Further, it is necessary to monitor the concentration while the workers are inside, to ensure a safe level of oxygen is maintained.
Conversely, measuring the concentration of oxygen in a sealed area can indicate, if the concentration is too high, that a seal is leaking. For example, the sealed space between window panes of a thermally efficient window may be filled with a gas of low thermal conductivity, such as argon. If a relatively high concentration of oxygen is present in the sealed space, it indicates that the argon is escaping and air is entering the space. In this type of application, the concentration of oxygen must be non-invasively measured, to avoid adversely affecting the seal.
Oxygen absorbs light in the 760 nanometer spectral region. Oxygen's absorption spectrum in this region consists of a number of narrow absorption lines that together are referred to as the "atmospheric oxygen A-band." A known prior system for measuring the concentration of oxygen uses a diode laser as a light source, and determines the absorption coefficient of one of the spectral lines of the atmospheric A-band as a function of the laser's wavelength. As long as the laser produces light with a wavelength that is the same as or close to one of the absorption lines of oxygen, the system can determine the concentration of oxygen based on the absorption coefficient of the spectral line.
One of the problems with this type of system is that the diode laser is unreliable. As the laser ages it experiences unpredictable changes in both operating temperature and current required to tune the laser to a given wavelength, which may result in changes in the wavelength or variations in the intensity of the light it produces. To compensate for these changes, the laser system may include a complex feedback mechanism and/or it may require a skilled technician to operate it. This makes the system expensive both to build and to use.